Some scientists think there may be more than one universe. This idea is called the multiverse hypothesis. This image is of a universe with different physical constants from our own.

The Universe

The study of the universe is called
cosmology
. Cosmologists study the structure and changes in the present universe. The
universe
contains all of the star systems, galaxies, gas, and dust, plus all the matter and energy that exists now, that existed in the past, and that will exist in the future. The universe includes all of space and time.

Evolution of Human Understanding of the Universe

What did the ancient Greeks recognize as the universe? In their model, the universe contained Earth at the center, the Sun, the Moon, five planets, and a sphere to which all the stars were attached. This idea held for many centuries until Galileo's telescope helped people recognize that Earth is not the center of the universe. They also found out that there are many more stars than were visible to the naked eye. All of those stars were in the Milky Way Galaxy.

In the early 20th century, an astronomer named Edwin Hubble (
Figure
below
) discovered that what scientists called the Andromeda Nebula was actually over 2 million light years away — many times farther than the farthest distances that had ever been measured. Hubble realized that many of the objects that astronomers called nebulas were not actually clouds of gas, but were collections of millions or billions of stars — what we now call galaxies.

(a) Edwin Hubble used the 100-inch reflecting telescope at the Mount Wilson Observatory in California to show that some distant specks of light were galaxies. (b) Hubble’s namesake space telescope spotted this six galaxy group. Edwin Hubble demonstrated the existence of galaxies.

Hubble showed that the universe was much larger than our own galaxy. Today, we know that the universe contains about a hundred billion galaxies — about the same number of galaxies as there are stars in the Milky Way Galaxy.

Summary

The universe contains about a hundred billion galaxies.

The idea of a universe has changed through human history.

Edwin Hubble saw the enormity of space and determined that there was much more than our own galaxy.